This invention relates to electrodes, and, more particularly, to hollow cathode electrodes for use in fluorescent lamps.
Conventional fluorescent lamp electrodes contribute to darkening of the ends of the lamp. This phenomenon reduces the luminous efficacy of the lamp as a function of lamp running time. The material deposited on the walls of the lamp is typically a mixture of evaporated barium and other material sputtered from the electrode. Additionally, this phenomenon also limits the life of the lamp because of the eventual removal of emission mix from the electrodes so that starting becomes difficult for the lamp ballast.
Hollow electrodes are generally operable in the so-called hollow cathode discharge mode of operation and such hollow electrodes offer several advantages. First, these electrodes generally produce less darkening of the lamp ends and a longer lamp life with better lumen maintenance over the life of the lamp, than do lamps employing heated filament cathodes. The reason for this advantage is the containment of sputtered and evaporated material within the hollow portion of the electrode. For the case of barium emissive mixes, this is especially useful since it provides a low work function and a correspondingly low electrode fall voltage, which, in turn, reduces sputtering. The work function is a measure of the energy required for removal of electrons from the surface of the electrode.
Simple cylindrical sleeve electrodes having emission mixes disposed on their interior surfaces are known in the prior art. In particular, A. Bouwknegt and A.G. Vanderkooi have reported on such structures in Proceedings of the First International Conference on Gas Discharges, 1971, pg. 217. In this paper, the authors indicated that desirable goals would include 50,000 lamp starts with 12,000 hours of average lamp life. However, using only the simple hollow electrode, the above authors indicated that such lamps operate for only 12,000 hours without serious depreciation, the lamp having incured only 6,000 starts. In contrast, conventional presentday fluorescent lamps generally exhibit an average life in excess of approximately 20,000 hours.
Many of the problems associated with poor starting and shortened lamp life are related to the simple cylindrical electrode employed. In particular, it is first noted that the opposite electrode surface from that which holds the emission mix, namely, the outside of the cylinder, is exposed for thermal radiation. Since emission requires a temperature of approximately 800.degree. C. to approximately 900.degree. C., this results in a considerable rate of energy loss. The cathode fall voltage must increase and/or current must increase to supply this energy at the electrode surface. Additionally, the starting of the discharge involves some deterioration of the end of the cylinder. Furthermore, lamp starting tends to destroy emission mix and the high operating temperature can lead to punch-through of the cylinder.
It should also be noted that it is highly desirable that any electrode, particularly fluorescent lamp electrodes, be configured in a structural arrangement which promotes rapid, facile and economical assembly. Accordingly, improvements in the conventional hollow cathode electrode design should not generally preclude rapid and economical manufacture.